The present invention relates to a tapping apparatus and method.
To cut an internal thread in a workpiece, a numerically controlled machine tool such as a machining center and a numerically controlled (NC) tapping machine are used. Such a machine tool includes a spindle, to which a tap is attached, a spindle motor for rotating the spindle and a feed motor for moving the spindle in the axial direction. When tapping a workpiece, the spindle motor and the feed motor are controlled such that the rotational phase of the spindle changes in synchronization with the axial position (feed position) of the spindle.
A tapping procedure includes a cutting process and a withdrawing process. In the cutting process, a tap is moved axially from a start position, which is separated from the workpiece by a predetermined distance, to an end position, at which the tap is located in the workpiece. In the withdrawing process, the tap is moved from the end position to the start position. The cutting process includes an acceleration step, a constant speed step and a deceleration step. In the acceleration step, the rotational speed of the spindle is increased from zero to the maximum level. In the constant speed step, the rotational speed of the spindle is maintained at the maximum level. In the deceleration step, the rotation speed of the spindle is decreased from the maximum level to zero. The withdrawing process also includes an acceleration step, a constant speed step and a deceleration step.
During a tapping procedure, the load on the tap can be excessive due to the expiration of the life of the tap T or to swarf trapped between the thread and the tap. An excessive cutting load may damage the tap and the internal thread formed in the workpiece. Japanese Unexamined Patent Publication No. 4-30910 discloses a numerically controlled machine tool that discontinues tapping when a tap receives an excessive cutting load.
In the machine tool of the above publication, load acting on the spindle motor is detected during a tapping procedure. Also, whether the load is greater than a predetermined acceptable level is detected. When the load surpasses the acceptable level, the spindle motor and the feed motor are stopped, which stops rotation and axial movement of the spindle. Thereafter, the spindle motor and the feed motor are started, which rotates and axially moves the spindle such that the tap is returned to the start position. As a result, the tap and the workpiece are prevented from being damaged.
The level of the electric current supplied to the spindle motor varies in accordance with load acting on the spindle motor. Therefore, in the machine tool of the publication, the load acting on the spindle motor is detected based on the level of the current supplied to the spindle motor.
Load acting on the spindle motor includes not only the cutting load due to cutting of the workpiece but also inertial load due to the inertial force of the rotating members such as the rotor of the spindle motor and the spindle. The inertial load, which is generated when the spindle is being accelerated and the decelerated, is considerably greater than the cutting load. When the spindle is rotating at a constant speed, little inertial load is generated and the load acting on the spindle motor consists mainly of cutting load.
In the machine tool of the publication, the load acting on the spindle motor is detected only when the spindle is rotating at a constant speed. Specifically, the load is detected only from immediately before the acceleration step is finished to when the constant speed step is finished. Therefore, the cutting load acting on the tap is substantially accurately detected when the spindle is rotating at a constant speed.
However, when measured with a measuring instrument, the cutting load torque increases during the deceleration step and is maximized at the end of the deceleration step. The machine tool of the publication cannot detect an excessive cutting load during the deceleration step.
When a workpiece made of a low hardness metal such as aluminum is tapped, the cutting load is small. Therefore, the rotation speed of the spindle and the feeding speed can be increased, which reduces the time required for tapping. In this case, the spindle starts being decelerated after a relatively short period has passed from when the acceleration step is finished. That is, the time during which the spindle is driven at a constant speed is significantly short. Thus, it is extremely difficult for the machine tool of the publication to detect the cutting load acting on the tap.
Accordingly, it is an objective of the present invention to provide a tapping apparatus and a tapping method that continuously and accurately detect the cutting load acting on a tap.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a tapping apparatus for cutting an internal thread in a workpiece with a tap is provided. The tapping apparatus includes a spindle to which the tap is attached, a spindle motor for rotating the spindle, a detection device for detecting the load acting on the spindle motor during a tapping procedure and a computer. The computer compares the load detected by the detection device with a predetermined referential load to detect a relative cutting load acting on the tap.
The present invention also provides a tapping method for cutting an internal thread in a workpiece with a tap. The method includes rotating a spindle to which the tap is attached by a spindle motor to perform a tapping procedure, detecting the load acting on the spindle during the tapping procedure and comparing the detected load with a predetermined referential load to detect a relative cutting load acting on the tap.